Flip chip semiconductor devices utilize solder bumping technology as a method of providing interconnection from a semiconductor die, also known as a chip, to a package or substrate. Rather than employing lead members, a in wire bonding and TAB (tape automated bonding) techniques, flip chip devices have solder bumps formed on bond pads of a semiconductor die. These solder bumps are then coupled directly to a substrate such as a printed circuit (PC) board. In order to couple the solder bumps directly to the substrate, the die must be turned over to a face-down position, thus the term "flip chip."
In fabricating a flip chip device, several processing steps are required after traditional semiconductor die fabrication. Solder bumps are not generally formed directly on bond pads of a semiconductor die. Instead, one or more metals is deposited onto the bond pads to form a terminal pad. As an example of conventional terminal pad metallurgy, a series of chrome, copper, and gold layers is deposited onto the bond pads prior to actual solder bump formation. The terminal pad metallurgy is used to prevent contaminants such as chlorine from attacking the bond pad. Terminal pads also provide a better solderable surface than traditional aluminum bond pads. Solder does not effectively wet aluminum surfaces; therefore, solder balls formed on aluminum bond pads generally have very poor adhesion. Materials used to form terminal pads on the bond pads can be chosen to serve both as a barrier to contamination and as a solder-wettable surface.
After any layers of terminal pad metallurgy have been formed on a flip chip device, solder is selectively evaporated onto the device. A shadow mask, usually made of metal, is positioned over the device during the evaporation process. The mask is provided with openings which correspond to the bond pad configuration of the device. Solder is deposited onto the mask and through the openings onto the bond pads, or on the terminal pads if present. Following deposition, the evaporated solder on the bond pads of the device is reflowed. Reflowing the solder causes the solder to soften and take on a semispherical shape due to surface tension forces, much like a water droplet on a glass or plastic surface. Due to this semi-spherical shape, solder bumps are also commonly referred to as solder balls. The solder bumps are then cooled, such that a metallurgical bond is created between the solder and the bond pad or terminal pad metallurgy.
There are several disadvantages with existing flip chip device fabrication processes, such as that described above. A significant disadvantage of the process is cost. The evaporation of solder is expensive due to equipment costs and the lengthy amount of time required to deposit solder onto the device. In addition, evaporation chambers require frequent cleaning to remove solder which has been deposited onto chamber walls. The chamber cleaning process is not only costly, but is also environmentally and physically hazardous. Another disadvantage with existing solder bump techniques is that shadow masks are required. The masks, which are often made of molybdenum, are expensive and must be replaced periodically. After each use, the masks must be cleaned to remove solder from the mask. Depending on the type of mask cleaning process used, the mask itself might be etched, resulting in changes in the size of the openings through which solder is deposited. Furthermore, differences in the coefficient of thermal expansion between the mask and the semiconductor device create alignment problems. The mask will expand at elevated temperatures, including evaporation temperatures, at a different rate than the device. Therefore, openings in the mask may no longer align to the bond pad locations. Yet another disadvantage with existing solder bump processes is that the composition of the solder bump is difficult to control. Although a solder of a specific composition can be evaporated onto bond pads, reflowing the solder results in a composition differential between various portions of the solder bump. Because the partial pressure of lead is greater than the partial pressure of tin, more lead will end up in the bottom portion of the bump (i.e. the portion adjacent the bond pad) than at the top of the bump. As a result, it is difficult to control and optimize solder bump composition.